A graphene membrane is a single-atomic-layer-thick layer of carbon atoms, bound together to define a sheet. The thickness of a single graphene membrane, which may be referred to as a layer or a sheet, is approximately 0.2 to 0.3 nanometers (nm). In some embodiments, multiple graphene layers can be formed, having greater thickness and correspondingly greater strength. Multiple graphene sheets can be provided in multiple layers as the membrane is grown or formed. Or multiple graphene sheets can be achieved by layering or positioning one graphene layer on top of another. For all the embodiments disclosed herein, a single layer of graphene or multiple graphene layers may be used. Testing reveals that multiple layers of graphene maintain their integrity and function as a result of self-adhesion. This improves the strength of the membrane and in some cases flow performance. In most embodiments, the graphene membrane having 2 or more layers is 0.5 to 2 nanometers thick. The carbon atoms of the graphene layer define a repeating pattern of hexagonal ring structures (similar to benzene rings constructed of six carbon atoms), which form a honeycomb lattice of carbon atoms. An interstitial aperture is formed by each hexagonal ring structure in the sheet and this interstitial aperture is less than one nanometer across. Indeed, skilled artisans will appreciate that the interstitial aperture is believed to be about 0.23 nanometers across at its longest dimension. Accordingly, the dimension and configuration of the interstitial aperture and the electron nature of the graphene precludes transport of any molecule across the graphene's thickness unless there are perforations. This dimension is much too small to allow the passage of either water or ions.
Currently, perforated graphene is considered a promising material for achieving molecular filtration. A perforated graphene high-flux throughput material provides significantly improved filtration properties, as opposed to polyimide or other polymeric material filtration materials.
Molecular filtration requires pores to be sized at the molecular level. It is desired for the relevant pore size to range from sub-nanometer (about 0.5 nm) to approximately 20 nanometers in size. However, it has been found to be very difficult to obtain such a size range with conventional tools, especially when trying to obtain perforated graphene over large areas (greater than mm2) needed for filtration. Indeed, for filtration applications, pore size must be tightly controlled to achieve proper rejection of the target species. When using graphene as the filter medium, the density of and the size of the holes in the graphene must be such that the material is not significantly weakened. But neither should the flow through the graphene material be significantly reduced. It has also become apparent that controlling the chemistry of the pores is important, especially in filtration applications where transit through the pores will be affected by the functional groups lining the edge of the pores or apertures.
One method attempted to obtain perforated graphene is referred to as a subtractive method. The subtractive method makes a periodic array of uniform holes in graphene by using a block co-polymer that can be developed to form an etching mask with a periodic array of holes. This is sometimes referred to as a top-down perforation methodology. In such an embodiment, an etch mask of anodic aluminum oxide (AAO) membrane or block copolymer (BCP) film is utilized wherein O2 plasma is directed through the mask so as to etch a sheet of graphene material. Another approach is template-free energy bombardment. This can be done by ion bombardment of highly ordered pyrolitic graphite (HOPG) or with atmospheric plasma. These methods are problematic in that the length scale of the holes and their spacing is on the tens of nanometers (i.e., greater than 20 nm) scale. This precludes use of the material for molecular filtration of small molecules and limits the use of electronics and optics to applications requiring a band gap of approximately 0.1 eV.
Another approach to forming perforated graphene is referred to as a bottom-up solution. This methodology requires surface-assisted condensation of small molecules, such as in Ullman-type synthesis. Assemblies at interfaces utilizing a solvent and HOPG interface have also been attempted, along with cylco-proparene. However, such approaches have not been found to be conducive for manufacturing processes.
Perforated graphene has a number of possible applications including, for example, use as a molecular filter, use as a defined band gap material, and use as an electrically conductive filler material with tunable electrical properties within polymer composites. Although a number of potential uses for perforated graphene exist, as discussed above, there is no reliable way of introducing holes, or pores, to graphene in the size range of about ten nanometers (10 nm) and under, and particularly about five nanometers (5 nm) and under. Multi-step but laborious lithography techniques can be used to fabricate holes greater than about twenty nanometers in size. However, no techniques are presently suitable for fabrication of perforated graphene on the scale of square meters per minute or more.
In summary, the prior art has not been able to provide a methodology for creating uniformly sized and spaced perforations in graphene. Overcoming such a problem can enable a variety of applications in filtration, optics, electronics and structural and thermal materials. Therefore, there is clearly a need in the art for a way to generate a perforated material of the correct pore size and the number of pores in a given area for use in molecular filtration and other applications.